1. Field of the Invention
The present invention relates to a thin-film forming apparatus which is employed for the fabrication of a display device having an EL (electroluminescence) element (hereinbelow, termed “EL display device”), and to a method of fabricating the EL display device as employs the thin-film forming apparatus.
2. Description of the Related Art
In recent years, researches have been vigorously made in EL display devices each of which has an EL element as a spontaneous emission type element. In particular, notice has been taken of an organic EL display device which employs an organic material as an EL material. The organic EL display device is also called the “organic EL display (OELD)” or “organic light emitting diode (OLED)”.
Unlike a liquid crystal display device, the EL display device is of spontaneous emission type and therefore has the merit of involving no problem on a view angle. In other words, the EL display device is more suitable than the liquid crystal display device as a display which is used outdoors, and its application in various forms has been proposed.
The EL element has a construction in which an EL layer is sandwiched in between a pair of electrodes, and in which this EL layer has a multilayer structure ordinarily. Typically mentioned is the multilayer structure of “hole transporting layer/light emitting layer/electron transporting layer” proposed by Tang et al., Eastman Kodak Company. The multilayer structure exhibits a very high emission efficiency, and most of the EL display devices being currently under research and development adopt this structure.
Herein, a predetermined voltage is applied to the EL layer of the above structure by the pair of electrodes, whereby light is emitted owing to the recombination of carriers taking place in the light emitting layer. There are two sorts of schemes for the light emission; a scheme wherein the EL layer is formed between two stripe-like electrodes disposed orthogonally to each other (simple matrix scheme), and a scheme wherein the EL layer is formed between pixel electrodes connected to TFTs and arrayed in the shape of a matrix, and a counter electrode (active matrix scheme).
Meanwhile, the EL materials of the hole transporting layer, the light emitting layer, etc. are broadly classified into two; a low molecular material, and a high molecular material. While materials mainly including Alq3 have been known for a low-molecular light emitting layer for a long time, a high-molecular (polymeric) light emitting layer has been noticed especially in Europe in recent years. Typically mentioned are PPV (polyphenylene vinylene), PVK (polyvinyl carbazole), polycarbonate, etc.
The reasons why the high-molecular EL material is noticed are the points that it can be formed into the layer by a simple method of forming a thin film, such as spin coating process (also termed “solution application process”), dipping process, printing process or ink jet process, and that it is higher in the thermal stability as compared with the low molecular material.
Usually, the low-molecular EL material is formed into the layer by vacuum evaporation. That is, ordinarily the EL material is successively stacked without breaking a vacuum in a vacuum evaporator. Besides, an electrode of small work function is employed as the electrode acting as the cathode of the EL element, and also this cathode is ordinarily formed in succession to the EL material.
The EL material is extraordinarily liable to oxidize, and the oxidation is readily promoted even by the presence of a slight water content till the degradation of this EL material. In case of forming the EL element, therefore, the surface of the anode thereof being the lowermost layer is first preprocessed to eliminate moisture etc., whereupon the EL material and the cathode are successively formed on the anode without breaking a vacuum. On this occasion, the EL material and the cathode are sometimes deposited on the selected parts of the anode by employing a shadow mask or the like, and all the processing steps are performed in an evacuated processing chamber even in such a case.
This holds true also of the high-molecular EL material. Even in case of the spin coating process etc. which are not a thin-film forming expedient in a vacuum, it is important for the suppression of the degradation of the EL material that the EL material is prevented from being exposed in the atmospheric air which contains moisture.